During Episodes IV-VI, were Darth Vader and Darth Sidious the only Sith alive? I know after VI Darth Caedus and all those other Sith are born, but during those times were there any unmentioned members of the Order of Sith Lords?

That said, if you allow EU content, there's the Lost Tribe of the Sith, on Kesh- whose leaders called themselves Dark Lords of the Sith. Most of the other dark side factions present in the galaxy didn't call themselves Sith, even if some of their knowledge was Sith-derived. Like the Prophets of the Dark Side, and the Sorcerers of Tund, both Sith splinter factions.

Well, if we're getting into EU, I doubt Palpatine and Vader would consider those others Sith due to ideological differences, and would probably try to destroy them if they'd know of their existence, especially if they wouldn't integrate into Palpatine's plan the way the Prophets did.

Well, if we're getting into EU, I doubt Palpatine and Vader would consider those others Sith due to ideological differences, and would probably try to destroy them if they'd know of their existence, especially if they wouldn't integrate into Palpatine's plan the way the Prophets did.

The last Sorcerer, Rokur Gepta, wiped out the rest of the order with a superweapon after learning their secrets- I suppose he could have been sent to them by Palpatine to do exactly that.

"Movie canon" doesn't confirm anything. Yoda, an old coot who couldn't notice a kid he's known for 14 year was turning into an Jedi-hating monster randomly blurts out that there are only two Sith based on a rumor he heard 800 years ago.

"I don't read that stuff. I haven't read any of the novels. I don't know anything about that world. That's a different world than my world. But I do try to keep it consistent. The way I do it now is they have a Star Wars Encyclopedia. So if I come up with a name or something else, I look it up and see if it has already been used. When I said [other people] could make their own Star Wars stories, we decided that, like Star Trek, we would have two universes: My universe and then this other one. They try to make their universe as consistent with mine as possible, but obviously they get enthusiastic and want to go off in other directions."GL
"There are two worlds here," explained Lucas. "There's my world, which is the movies, and there's this other world that has been created, which I say is the parallel universe—the licensing world of the books, games and comic books. They don't intrude on my world, which is a select period of time, [but] they do intrude in between the movies. I don't get too involved in the parallel universe."-GL